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Exterior Shade Performance Analysis
At Mermet, we have started to make a change in the way we represent the performance data of our fabric to architects and designers. Although we will also report the measured fenestration values of our fabric by itself, for product-to-product comparison, we recognize that our intended audience wants to know how that shade is going to impact the performance of the building conveyed in the terms they are used to for US code compliance. This information is being integrated into the redesign of our website, product brochure, and in the future development of our sample cards.
GLASS* + 3% E-SCREEN CHAR/CHAR FABRIC / GLASS* + 3% E-SCREEN WHITE/WHITE FABRICSYSTEM SHGC = 0.33 % Improvement = 13%
SYSTEM VLT = 3% LSG = 0.09 / SYSTEM SHGC = 0.16 % Improvement = 59% SYSTEM VLT = 12% LSG = 0.75
3% E-SCREEN CHAR/CHAR FABRIC + GLASS* / 3% E-SCREEN WHITE/WHITE FABRIC + GLASS*
SYSTEM SHGC = 0.06 % Improvement = 84% SYSTEM VLT = 2% LSG = 0.33 / SYSTEM SHGC = 0.07 % Improvement = 82% SYSTEM VLT = 9% LSG = 1.28
Rs – Solar Reflection / As – Solar Absorption / Ts- Solar Transmission / = 100% Solar Energy
* Glazing performance is based off using Mermet fabrics with common Double Glazing (6 mm / 1/2” air / 6 mm) glass with low E on surface #2 (Guardian SN68, Viracon 1-2 M, PPG Solarban 60).
In the example used, the glazing performance without shades is SHGC (.38) and Tv (68). Variances in VLT reduction and SHGC improvement will occur with the use of project specific glass types and should be calculated using LBNL Window software.
Example Performance
Fabric Measured Data
Rs / As / Ts / Tv3% E-Screen White/White / 74 / 9 / 17 / 15
3% E-Screen Char/Char / 4 / 91 / 5 / 5
*Measured internally on a Perkins Elmer Spectrophotometer
Glazing Makeups Calculated Data / SystemSHGC / VLT / SHGC % Improvement
SN 68 (#2) on clear – 1” Double glazing IGU
– no shade / 0.38 / 0.67 / baseline
SN 68 (#2) on clear w/ 3% E-Screen White/White - Interior / 0.16 / 0.12 / 59%
SN 68 (#2) on clear w/ 3% E-Screen White/White - Exterior / 0.07 / 0.09 / 82%
SN 68 (#2) on clear w/ 3% E-Screen Char/Char - Interior / 0.33 / 0.03 / 13%
SN 68 (#2) on clear w/ 3% E-Screen Char/Char - Exterior / 0.06 / 0.02 / 84%
*Calculated using LBNL Window v7.3 (CGDB)
Heat Blockage Factors: Solar Heat Gain Coefficient | SHGC
Solar Heat Gain Coefficient indicates how effective a building envelope system is at blocking heat from solar energy. To calculate a product’s SHGC requires understanding the specific makeup of the entire exterior envelope system (window, glass and frame). Due to the level of complexity with possible glazing and frame combinations, Mermet has simplified the data by referencing SHGC as an approximate percent improvement when using our fabrics in combination with common window units. The higher the percentage, the more efficient the fabric is at blocking heat. For example, if a fabric has an SHGC percent improvement of 59% that means you multiply 0.59 by the SHGC of the project specific glazing (center of glass value). Subtract that number by the original SHGC of the glazing and you roughly have the overall SHGC for the system.
Glass SHGC - (Glass SHGC * % Improvement) = System SHGC.
Optical Factors: Visible Light Transmittance Tv | VLT
Visible Light Transmission (VLT) is commonly used to represent the amount of visible light that passes through the entire window system. Tv refers to the measured transmission of light through the fabric only. For example, to roughly calculate the overall system VLT (glass + fabric), simply multiply the Glass VLT by the fabric Tv.
Glass VLT * Fabric Tv = System VLT
Light-to-Solar Gain | LSG
LSG is the ratio of the visible light transmittance to the Solar Heat Gain Coefficient.
A higher LSG ratio means sunlight entering the room is more efficient for daylighting, especially for summer conditions where more light is desired with less solar gain. This ratio is the measurement used to determine whether the glazing system is “spectrally selective” (>1.25) as defined by the Department of Energy.
LSG=VLT/SHGC
Building Energy Savings Examples
Most building energy simulations have a hard time with shade attachments. The primary reason is the performance is annualized and does not accurately account for seasonal operation. Currently, the Attachment Energy Rating Council (AERC) is in the process of creating default calculation practices and usage rules so that all products can be properly calculated as part of the glazing system and properly compared for building energy impact.
Key Observations
· Dark colored fabrics offer better view through and glare control. One criticism is that when installed on the interior, the solar heat absorption is reradiated into the interior making it a poor performer for thermal control. However, when installed on the exterior, you keep all of the benefits of view through and glare control, but the fabric absorption reradiates to the exterior and never enters the building.
o In the previous example used, note the 84% SHGC improvement when shades are installed on the exterior of a window, compared to only 13% when installed on the interior.
· Light colored fabrics offer superior daytime privacy and good natural ability to reflect solar energy. This is true if the shade is installed on the interior or exterior, however view through and glare control are compromised with light colored fabrics.
o Although the openness factor in the example product is low, it is important to note that the LSG value dramatically increased when the roller shade was installed on the exterior. This allows for more daylighting into the room while controlling thermal performance.